Multipassband and Wide-Stopband Filters via Independent Tuning of High-Order SSPP Modes

We propose a novel F-fold spoof surface plasmon polaritons (SSPPs) unit with independently tunable high-order rejection bands. The dispersion characteristics of the proposed F-fold SSPPs are analyzed. The results show that the cutoff frequencies of Modes 2 and 3 can be precisely adjusted through structural parameters, enabling independent control of the stopband position formed by these two modes, without affecting the fundamental mode (Mode 0) or Mode 1. Therefore, the passband number, bandwidth, and stopband width can be flexibly manipulated for the design of multiple passband filters. For verification, a tri-passband, quad-passband filter, and wide stopband filter are designed, fabricated, and measured. The wide stopband filter achieves a 547% increase in the stopband width compared to the tri-passband filter. Good agreement between the simulations and measurements confirms the feasibility of the proposed structure and design method.